1. Field of the Invention
The present general inventive concept relates to a heating roller used to fix a toner image, and more particularly, to a power control method and apparatus to control a heating roller, to supply an external source power to a heating resistor included in the heating roller.
2. Description of the Related Art
In an image forming apparatus, such as a printer or a copy machine, which forms an image of print data on a printing medium by using a developing material, such as toner, a toner image corresponding to the print data is fixed onto the printing medium, and the printing medium is then discharged out of the image forming apparatus, thereby obtaining a printed matter.
The image forming apparatus may use a heating roller having heating resistors. In this case, in order to perform a fixing operation, a surface temperature of the heating roller has to be maintained around a fixing target temperature, for example, 180° C.
The image forming apparatus is switched to a print mode when the image forming apparatus first receives a printing order after power is on, or when the image forming apparatus receives the printing order in a standby mode. Here, a time required after the printing order is received and before a first printed matter is discharged is referred to a first print out time (FPOT). In order to reduce the FPOT of the image forming apparatus, including the heating roller, the surface temperature of the heating roller has to reach the fixing target temperature in a rapid manner.
FIGS. 1A-1C illustrate a power control principle of a conventional heating roller. If a resistance of a heating resistor is determined in proportion to a heating roller's temperature equal to or less than a threshold temperature, and a voltage (Vin) 110 illustrated in FIG. 1A is applied to the heating resistor, then a current (Ir) 120 illustrated in FIG. 1B flows through the heating resistor.
If the current (Ir) 120 is gradually decreased until the heating roller's temperature reaches the threshold temperature, the power control principle for the conventional heating roller has a drawback in that a circuit may be damaged due to an electric shock because an excessive current may flow through the heating resistor when power begins to be supplied to the heating resistor. In this case, a high current may flow through the heating roller in the form of an alternating current, thereby deteriorating a flicker characteristic. The flicker characteristic can be defined as a phenomenon in which power supplied to a peripheral circuit is temporarily weakened.
A threshold resistance that represents a resistance of a heating resistor at a threshold temperature is determined intrinsically. Here, the lower the threshold resistance is used, the more the power can be supplied to the heating resistor. Thus, the surface temperature of the heating rollers can be rapidly increased. However, when a heating resistor having a lower threshold resistance is used, a higher current flows through the heating resistor when power begins to be supplied to the heating resistor, thereby causing the aforementioned problems. Eventually, in the conventional power control principle for a heating roller, a heating resistor has to have a sufficiently low threshold resistance, and thus, there has been a limit in reducing a time required for increasing a surface temperature of the heating roller up to a fixing target temperature STt.
Furthermore, if the image forming apparatus receives a printing order after the image forming apparatus is turned on, the heating roller can be heated after a control unit (not illustrated), which controls overall tasks performed in the image forming apparatus, for example, a central processing unit (CPU) of the image forming apparatus, is initialized. Therefore, the aforementioned problem that there is a limit in reducing a warm-up time for printing becomes more apparent when the image forming apparatus receives the printing order before the control unit (not illustrated) is initialized.
According to the conventional power control principle, as illustrated in FIG. 1C, a heating roller is heated until a surface temperature thereof reaches a fixing standby temperature STr, for example, 160° C., that is, during time t=t1˜t2. In addition, after the surface temperature of the heating roller reaches the fixing target temperature STr, a pressure roller co-rotates with the heating roller until surface temperatures of the heating roller and the pressure roller reach the fixing target temperature STt, that is, during time t=t2˜t3.
Meanwhile, during a time after the surface temperature of the heating roller reaches the fixing target temperature STt and before the image forming apparatus receives the printing order, that is, during time t=t3˜t4, no power is supplied to the heating resistor. Further, if the image forming apparatus receives the printing order after the surface temperature of the heating roller reaches the fixing target temperature STt (t=t4), the heating roller is heated such that the surface temperature thereof is maintained at the fixing target temperature STt.
In this case, the surface temperature of the heating roller is not decreased right after the power stops to be supplied to the heating roller (t=t3+), but is increased up to a specific temperature STos, and thereafter is decreased. Likewise, the surface temperature of the heating roller is not increased right after the power begins to be supplied to the heating roller (t=t4+), but is decreased to a specific temperature, and thereafter is increased.
Accordingly, in the conventional power control principle for a heating roller, once the surface temperature of the heating roller reaches the fixing target temperature STt, a roller power is no longer supplied to the heating resistor. Thus, if a printing medium is fed a long time after the power stops to be supplied to the heating resistor, a toner image cannot be fixed onto the printing medium in a stable manner. This becomes more apparent when the printing medium is fed at a low temperature, such as, a room temperature.